The joint observation of GW170817 and GRB 170817A allowed for a simple, but very powerful, test of the speed of gravity. The peak of gravitational-wave emission
and the gamma-ray burst arrived at the Earth within seconds of each other. If one assumes that these were emitted from the source at roughly the same time, then any deviation between their arrival times at the Earth would be due to differences in the speed of the two messengers. In the
absence of any observable delay, a very strong bound on \(c_{\rm GW}\)  can be placed. Indeed, this observation constrained \(\delta c_{\rm GW}\)  in the 10 - 1000Hz frequency band to \(\lesssim 3\times 10^{-15}\)  [1]}.
